The present invention relates to a high frequency antenna for an apparatus measuring the nuclear magnetic resonance. It finds its application more particularly in the medical field where the measurement of the nuclear magnetic resonance is used for forming tomograms.
1. Field of the Invention
An apparatus for measuring the magnetic resonance of the nuclei of a body comprises means for subjecting this body to an intense continuous magnetic field. It also generally comprises means for applying continuous magnetic field gradients along three axes for frequency discriminating different sections of the body. With this continuous or pseudo-continuous magnetic induction established, the body to be examined is subjected to a high frequency alternating magnetic energization. The frequency of this energization is related to the intensity of the continuous magnetic field and to the gyromagnetic ratio of the nuclei of the body. It is in these conditions of concordance that the phenomenon of nuclear magnetic resonance occurs. Then the energization is stopped and the de-energization wave produced by the nuclei when they return to their state of equilibrium is measured.
2. Description of the Prior Art
The sensor which receives the de-energization wave is the same device as the one which serves for applying the high frequency energization: it is called high frequency antenna. A duplexer allows the energization to be oriented correctly towards the antenna, and the de-energization wave towards the detection means. Different types of antennae are known. A particular pattern of antenna has been developed, saddle antenna, whose advantage is that it is readily adapted to the study of stretched out bodies such for example as human bodies. In these antennae, the radiating element comprises turns through which currents flow. Another type of antennae comprises conductors disposed over a fictitious cylinder, parallel to and externally of the body to be examined. These conductors are connected at their ends by capacitors to a conducting metal screen which envelops the fictitious cylinder. The capacitors are tuned so that the conductors form, with the screen resonating line elements resonating at the frequency of the high frequency electric signal which is applied thereto. An example of such a structure is described in the European patent application No. 0 073 375 filed on the Aug. 12, 1982. The conductors are there arranged in pairs. The conductors of one pair are diametrically opposite each other on the fictitious cylinder. At each half wave of the energization signal, the current which flows in a conductor of one pair is opposite in direction to that which flows in the paired conductor. For reasons of homogeneity, each conductor may be replaced by a group of adjacent conductors connected together at their ends.
An antenna element comprises then two conducting half elements: each half element corresponds, in this embodiment, to a single conductor or to a group of adjacent conductors. In order to improve the gain of the antenna, and consequently the signal to noise ratio during detection of the de-energization wave, it is necessary for the intrinsic resonance frequency of each of these half elements to be the same. So that the half elements have the same resonance frequency it is necessary to adjust their tuning capacitors concurrently: two half elements in fact behave with respect to each other like two coupled resonators. Taking into account the energization frequencies used (a few mega hertz) and the tolerable divergences between the over voltage frequencies of each of the half elements and the energization frequencies, these adjustments are delicate. The problem is all the more complicated since an antenna generally comprises more than one antenna element. It often comprises two. In this latter case, it is then necessary to adjust four capacitors.
Despite everything, this difficulty could be overcome if it was sufficient to adjust the capacitors once and for all in the factories. An additional drawback is due to the fact that the bodies examined are not comparable with each other. They induce in the half elements of each antenna element reactive impedances which differ from one case to another. The antennae are then out of tune: for each body (for each patient) the adjustment must be made again. This latter adjustment is not carried out in the factory. In addition, in most cases, the frequency tuning of all the resonators formed by the conducting half element is not even undertaken. The operator makes shift with degraded operation prejudicial to the quality of the tomogram images produced.
Moreover, it appears necessary to impedance match the antenna with the supply and reception circuits to which it is connected. This requirement goes hand in hand with the need to adjust the power transmitter to the antenna by these supply circuits. The lack of matching present at transmission is also present at reception. Now, as has already been mentioned, the bodies to be examined (the patients) do not ressemble each other. From one situation to another the impedance presented by these bodies to the hiqh frequency magnetic energization is different. In short, the load presented by the antenna, or the energy consumption varies. This is why it is advisable to adjust the power and match the impedance.
In short, if the load presented by the body to be examined, and seen through the antenna from the supply means, is z=a+jb, it may be considered that frequency tuning is obtained when b is o and that power matching is obtained when a is equal to the internal resistance of the supply-reception means. It is because all these adjustments are difficult, considering the complexity of an antenna system, that they are not even undertaken.